In an electrical utility grid consumers can usually consume electric power in an uncontrolled manner. Since hardly any energy is stored in the grid, there can be no imbalance between the power produced and the power consumed. Therefore, the momentary production of power shall match the momentary power consumption. Overproduction leads to an increase of the grid frequency beyond the nominal value (e.g. 50 or 60 Hz), since the conventional synchronous generators accelerate, while overconsumption will lead to a decrease of the grid frequency beyond the nominal value (e.g. 50 or 60 Hz), since the conventional synchronous generators will then decelerate.
In order to stabilize the frequency of the electrical grid, conventionally about 10% of the producers contribute to what is called “primary power control”. These producers, also referred to as “primary controllers”, increase power output when the frequency falls below the nominal value and decrease power output when it rises above the nominal value.
Conventionally, wind turbines do not contribute to primary control, firstly because wind turbines cannot normally increase their output power by command (as they normally operate at nominal load or, when operating at partial load, at an optimal working point), and secondly because the available wind power shall normally be entirely exploited. Exceptions have been proposed in which the output power of a wind turbine is increased upon decrease of the grid frequency below a certain frequency limit, and vice versa.
Generally, wind power adds an additional moment of grid instability because, with a significant fraction of wind power in a grid, not only the consumption is uncontrolled, but also the production by wind turbines. Even though wind forecasts enable the wind power production to be predicted with considerable accuracy on a long-term basis (at the level of hours), the wind speed normally fluctuates in an unpredictable manner on a short-term basis (at the level of minutes down to a few seconds). A wind turbine operating at partial load (i.e. when the wind speed is below the nominal wind speed of the wind turbine considered) will normally transform these wind-speed fluctuations into corresponding fluctuations of the amount of real power produced and supplied to the electrical grid. Only at wind speeds above nominal, when a wind turbine operates at nominal load, wind turbines normally control their output power to be constant at the nominal output power.
The consequence of fluctuating-power production by wind turbines on the grid stability depends on characteristics of the grid. In a large, stable grid a power fluctuation by a wind turbine or wind park will not produce any significant response in the form of a frequency fluctuation. Thus, such grids can cope with higher power variations. However, in a small isolated grid, or in weak grids, such a power fluctuation may produce a significant frequency fluctuation, so that high short-term power variations may cause stability and regulation problems in the grid.